Aquaculture system

ABSTRACT

An aquaculture system ( 10 ) comprising a building structure ( 11 ) defining a scaled environment ( 13 ) having an access system ( 17 ) incorporating an airlock ( 18 ), a culture tank ( 35 ) and water treatment means ( 41 ) for treating water in the culture tank for removing contaminants to maintain water quality. The system also, has air supply means ( 27 ) and temperature control means ( 31 ). A mechanical filtration system using an endless belt and biological filtration apparatus are also disclosed.

FIELD OF THE INVENTION

[0001] This invention relates to an aquaculture system. More particularly, the invention concerns a system for establishing a closed environment in which aquatic species can be cultured. The environment is closed in the sense that matter entering the environment is controlled to ensure that it is of a quality appropriate for the species under culture in order to avoid exposure of the species to disease and other harmful effects.

BACKGROUND ART

[0002] It is common to cultivate aquatic species as seafood in so-called fish farms where the aquatic species can grow and ultimately be harvested. Typically, the aquatic species are grown in tanks and dams. A particular difficulty with such an arrangement is that matter entering the aquatic environment in which the aquatic species are grown cannot be rigorously controlled. Consequently, disease can easily spread within the aquatic environment, with devastating results to the aquatic species under cultivation. Additionally, the environment is not controlled in order to provide optimum conditions for growth of the aquatic species under cultivation. Such conditions include maintenance of the water at an appropriate quality and temperature, and isolation of the environment in order to prevent contamination which could result in exposure of the aquatic species to disease and other harmful effects.

[0003] It is against this background that the present invention has been developed.

DISCLOSURE OF THE INVENTION

[0004] According to one aspect of the present invention there is provided an aquaculture system comprising a cell defining a closed environment, the cell having an access system incorporating an airlock for access to and from the closed environment, at least one culture tank accommodated in the closed environment, and water treatment means for treating water in the or each culture tank for removing contaminants therein to maintain the water of a quality appropriate to the species under culture.

[0005] Preferably, the system further comprises a water supply means for introducing water of a quality appropriate to the species under cultivation into the environment.

[0006] Preferably, the system further comprises air supply means for introducing air of a quality appropriate to the species under culture into the closed environment for various purposes such as ventilation and aeration of water contained in the or each culture tank.

[0007] Preferably, the system further comprises power supply means for introducing electrical power to the closed environment.

[0008] The water treatment means may comprise a plurality of filtration devices in a serial arrangement, water being extracted from the or each culture tank and contacted by the filtration devices for filtering thereof prior to being returned to the culture tank or tanks. The filtration devices may provide mechanical filtration and biological filtration.

[0009] The water treatment means may also include a sterilisation system for sterilising the water as appropriate.

[0010] The mechanical filtration may include a moveable structure incorporating a filter screen, means for contacting a portion of the filter screen with water requiring filtering, and means for periodically moving the filter screen for varying the portion thereof being contacted by the water.

[0011] With this arrangement, the moveable structure can be cleaned from time to time without the need to interrupt the filtration process, the cleaning operation being performed on the respective portion of the filter screen after it has been moved out of contact with the water being filtered.

[0012] Preferably, the moveable structure comprises an endless belt.

[0013] The filter screen may be of a uniform screening size throughout or it may be formed as a series of screen sections of various screening sizes so that different degrees of filtering can be performed during the filtration process.

[0014] The biological filtration may be adapted to receive water directly from the mechanical filter.

[0015] The biological filtration may be provided by a first biological filter and a second biological filter.

[0016] The first biological filter is preferably disposed beneath the mechanical filter to receive water directly from the mechanical filter.

[0017] The second biological filter may incorporate a protein skimmer.

[0018] The water treatment means may also include a sterilisation system for sterilising the water as appropriate.

[0019] According to another aspect of the invention there is provided a mechanical filtration apparatus comprising a moveable structure incorporating at least one filter screen, means for contacting a portion of the moveable structure with a liquid requiring filtering, and means for periodically moving the moveable structure for varying the portion thereof being contacted by the liquid.

[0020] The moveable structure preferably comprises a plurality of screen sections disposed along a direction of movement of the moveable structure. With this arrangement the filter screens can be sequentially moved into contact with the water.

[0021] The moveable structure may comprise an endless belt.

[0022] The means for contacting a portion of the moveable structure the liquid may comprise a receiving chamber into which the liquid can be delivered and from which the liquid can flow under gravity through the filter screen.

[0023] The mechanical filtration apparatus may further comprise a cleaning means for cleaning a respective portion of the moveable structure after that portion has moved out of contact with the liquid.

[0024] The cleaning means may comprise a washing system for applying a washing liquid such as water to the respective portion of the moveable structure. The washing liquid may be applied as a spray.

[0025] According to another aspect of the invention there is provided a biological filtration apparatus incorporating a protein skimmer, characterised in that the protein skimmer is disposed above a body of liquid undergoing biological filtration.

[0026] According to yet another aspect of the invention there is provided a biological filtration apparatus comprising a contact chamber, aeration means for introducing air into a body of liquid contained in the chamber, and a protein skimmer disposed above the body of liquid for collecting foam bubbles generated by aeration of the liquid, the protein skimmer comprising a body defining a tray for receiving foam bubbles.

[0027] The tray may surround a collar disposed about an opening extending through the body, the opening accommodating a delivery pipe of the aeration means with a clearance space therebetween.

[0028] The biological filtration apparatus may further comprise a further contact chamber, a further aerating means for introducing air into a body of liquid contained in the further contact chamber, and a further protein skimmer, wherein the two contact chambers are connected in series in a liquid flow circuit such that liquid undergoing biological filtration passes upwardly through one contact chamber and downwardly through the other contact chamber.

[0029] Preferably the liquid flow circuit has an inlet in one contact chamber and an outlet in the other contact chamber, the inlet and outlet each opening onto the respective contact chamber in a lower region thereof.

[0030] There may be a plurality of said biological filtration apparatus connected in series.

[0031] According to yet another aspect of the invention there is provided a water treatment system comprising a water circuit including a biological filter, a pump disposed on the downstream side of the biological filter, and an air-injected UV steriliser disposed on the downstream side of the pump.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The invention will be better understood by reference to the following description of one specific embodiment thereof as shown in the accompanying drawings in which:

[0033]FIG. 1 is a schematic plan view of an aquaculture system according to the embodiment;

[0034]FIG. 2 is a schematic perspective view of a primary filter used in the aquaculture system;

[0035]FIG. 3 is a further schematic perspective view of the primary filter;

[0036]FIG. 4 is a schematic sectional side view of the primary filter;

[0037]FIG. 5 is a schematic plan view of the primary filter;

[0038]FIG. 6 is a plan view of a mechanical filter incorporated into the primary filter of FIG. 2;

[0039]FIG. 7 is a side elevational view of the mechanical filter;

[0040]FIG. 8 is an end elevational view of the mechanical filter;

[0041] FIGS. 9 to 12 are schematic side elevational views of the mechanical filter at various stages of operation;

[0042]FIG. 13 is a schematic perspective view of a secondary filter forming part of the aquaculture system;

[0043]FIG. 14 is a schematic sectional side elevational view of the secondary filter, (shown without biomedia installed therein);

[0044]FIG. 15 is a plan view of the secondary filter; and

[0045]FIG. 16 is a view similar to FIG. 14 but on a larger scale and shown with biomedia in position.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

[0046] The embodiment shown in the drawings is directed to an aquaculture system 10 which is particularly, although not solely, suitable for culturing seafood such as fish from starter stock to a market size.

[0047] The aquaculture system 10 comprises a building structure 11 defining a sealed environment 13. The building structure 11 includes a sealed skin 15 (which may include thermally insulated walls) and an access system 17 incorporating an airlock 18. The access system 17 incorporates access doors 19, 21 between which the airlock 18 is located. Personnel, as well as all materials used in cultivation of the seafood, enter and leave the sealed environment 13 via the airlock 18. Such materials include food and chemicals required for growth of the starter stock and the seafood which mature therefrom, as well as ancillary materials such as ice and packaging. Additionally, the starter stock enter the sealed environment via the access system 17 and the seafood into which they mature leave the sealed environment via the access system.

[0048] A water supply means 23 is provided for introducing water of a quality appropriate to the seafood under cultivation into the sealed environment 13.

[0049] A power supply means 25 is provided for introducing electrical power into the sealed environment 13.

[0050] An air supply means 27 is provided for introducing air of a quality appropriate to the seafood under cultivation into the sealed environment 13 for ventilation purposes, as well as for various other purposes including aeration of water in which the seafood are cultured. The air introduced into the sealed environment 13 is filtered, generally to less than 5 μm The air supply means 27 also provides air for the airlock 18. The sealed environment 13 incorporates an outlet 29 through which stale air can leave the sealed environment.

[0051] A temperature control means 31 is provided for controlling the temperature within the sealed environment 13. In this embodiment, the temperature control means 31 incorporates a heat exchanger 33 located exteriorly of the sealed environment 13. With this arrangement, heat can be added to, and removed from, the sealed environment 13 as necessary.

[0052] A culture tank 35 is provided within the sealed environment 13 to accommodate water in which the seafood are cultured. In the embodiment illustrated in the drawings, there is only one tank 35. There may, however, be a plurality of tanks contained within the sealed environment 13, with the tanks being separate from each other. Indeed, it is advantageous for there to be a plurality of separate tanks so that any disease or other harmful effects to which seafood in one tank might be exposed would not necessarily be transmitted to seafood in other tanks.

[0053] The culture tank 35 has a top which is open to the sealed environment 13. The temperature of the water contained within the tank 35 can be regulated by control of the temperature of the sealed environment 13.

[0054] A water treatment system 41 is provided for treating water in the tank 35 for removing contaminants therein to maintain the water of a quality appropriate to the seafood under culture.

[0055] The water treatment system 41 includes a filtration system 43 for filtering contaminants from the water, and a sterilisation system 45 for irradiating the water to kill micro-organisms and pathogens that may be present in the water.

[0056] The filtration system 43 and the sterilisation system 45 are incorporated in a water circuit 47 having an inlet 49 and an outlet 51 communicating with the tank 35. The inlet 49 is incorporated at the bottom of the tank 35 such that water flows from the tank under the influence of gravity, and also circulates through the filtering system 43 on the outlet side of the filtering system 43 under gravity flow. A pump 53 is incorporated in the water circuit 47 on the outlet side of the filtering system 43 in order to return the water to the tank 35 through the outlet 51.

[0057] The water circuit 47 incorporates a discharge line 57 which extends to the exterior of the building 11 for discharging any surplus water and to enable the tank 35 and the water circuit 47 to be drained if necessary. The discharged water can be subsequently sterilised and used for other purposes, such as fertiliser.

[0058] Replenishment water is introduced into the water circuit 47 as necessary by way of the water supply means 23.

[0059] The sterilisation system 45 comprises an air injected UV steriliser 46 incorporated in the water circuit 47 on the downstream side of the pump 53. The air for the UV sterilisation process is injected into the water circuit 47 at injection point 48 immediately upstream of the UV steriliser 46.

[0060] An air lift system 59 is provided for gas stripping aerating and circulating water contained in the culture tank 35. The air lift system 59 receives air for operation thereof via an air supply line 61. The air lift system 59 operates to lift water upwardly from the lower region of the tank 35 so as to gas strip the body of water contained in the tank 35. The air lift system 59 also moves the water around the tank 35 for the purpose of carrying solid contaminants in the water to the inlet 49 of the water circuit 47 at the bottom of the tank for subsequent removal through the filtration system 43. The air lift system 59 also aerates the water through the introduction of the operating air into the water.

[0061] The filtering system 43 comprises a primary filter 63 and a secondary filter 65, the primary and secondary filters being connected in series.

[0062] The primary filter 63 incorporates a mechanical filter 67 and an optional first biological filter 69.

[0063] The primary filter 63 comprises a housing 71 incorporating a receiving chamber 73 into which water from the culture tank 35 enters via an inlet 75.

[0064] The mechanical filter 67 comprises a moveable structure in the form of an endless belt 81 passing around first and second end rollers 83, 85. End roller 85 is driven by way of a drive system 87 incorporating a motor and gearbox assembly 89.

[0065] The endless belt 81 incorporates a filter screen comprising a plurality of screen sections, there being four such screen sections 91, 92, 93 and 94 in this embodiment. The screen sections 91, 92, 93 and 94 comprise mesh screens, each of a different mesh size so that different degrees of filtration can be performed during the filtering process, as will be explained. In an alternative embodiment, the mesh screens may be of the same mesh size. In another variation, the filter screen may comprise a single mesh screen instead of a plurality of screen sections.

[0066] Filtration is performed at a filtration zone 95 which is associated with the receiving chamber 73 such that water to be filtered is contacted by the particular screen section located at the filtration zone as the water flows under gravity from the receiving chamber 73 through the screen section and into the first biological filter 69 located in the housing 71 below the mechanical filter 67.

[0067] The screen sections 91, 92, 93 and 94 can be sequentially moved into position at the filtration zone 95 by stepped movement of the endless belt 81 under the influence of the drive system 87. Each filter section remains at the filtration zone 95 for a prescribed duration, following which the drive system 87 is actuated to move the particular screen section out of the filtration zone and to bring the next filtering section into the filtration zone.

[0068] A washing system 97 is provided for washing each screen section after it has moved out of the filtration zone 95. The washing system 97 is incorporated in the drive system 87 and is provided with a spray bar 99 from which water is sprayed under pressure to wash each screen section of the endless belt 81 as it moves past the spray bar. Contaminants washed from the screen sections, as well as the washing water, are collected in collection unit 101 and discharged from the sealed environment 13 via the discharge line 57.

[0069] With this arrangement, the particular screen section located at the filtration zone 95 is periodically replaced with a clean screen section so as to ensure that the mechanical filtering process remains effective on an ongoing basis.

[0070] As previously mentioned, the various screen sections 91, 92, 93 and 94 in this embodiment have different screening sizes. With this arrangement, one screen section may remove coarse particulates while another may remove fine solids. This allows the water being circulated through the water circuit 47 to be screened to various degrees. Over an extended period of time, all of the water contained within the culture tank 35 would be contacted by all of the screen sections.

[0071] The first biological filter 69 comprises biomedia 102 accommodated in the housing 71 below the mechanical filter 67. While the biomedia may be of any appropriate type, in this embodiment it comprises plastic mesh wound into interspersed cylinders that fit one within another. The outer cylinders are constructed of a larger mesh than the inner cylinders. This greatly increases the size of the surface area on which bacteria can multiply and grow.

[0072] The housing 71 has an outlet 107 and a bottom wall 109 which slopes towards the outlet 107. The housing 71 is also provided with a selectively operable drain outlet 108.

[0073] The biological filtration process conducted in the first biological filter 69 is generally of an aerobic nature. The process is performed by a first nitrifying bacteria that converts ammonia (NH₃) to nitrite (NO₂), and a second nitrifying bacteria that converts nitrite (NO₂) to nitrate (NO₃). The bacteria performing such processes are from the Nitrosomonas and Nitrobacter species.

[0074] The secondary filter 65 comprises a second biological filter 111. The second biological filter 111 comprises a housing 113 incorporating an inlet 115 and an outlet 117.

[0075] The housing 113 comprises a first housing section 118 defining a first contact chamber 121, and a second housing section 119 defining a second contact chamber 122. The two housing sections 118, 119 are interconnected by a duct 120 which provides a flow path from the first contact chamber 121 to the second contact chamber 122. The first contact chamber 121 has a bottom wall 127 which slopes downwardly to the inlet 115. Similarly, the second contact chamber 122 has a bottom wall 129 which slopes downwardly to the outlet 117.

[0076] Aeration means 131 are provided for injecting air into each chamber 121, 122. Each aeration means 131 comprises a delivery pipe 133 which extends downwardly through the upper end of the respective housing section 118, 119 and terminates at a sparge 135 through which air is injected into the water within the respective chamber for the purpose of aerating the water. The intensity of the air injection strips the water of any fine particles.

[0077] A protein skimmer 137 is provided above each contact chamber 121, 122. Each protein skimmer 137 comprises a body 139 disposed above and in contact with the water contained within the respective contact chamber 121, 122. The body 139 comprises a central collar 141 defining an opening 143 through which the respective delivery pipe 133 extends. There is a clearance space 144 between the outer periphery of the delivery pipe 133 and the collar 141 such that air injected into the water by way of the aeration means 131 can escape through the clearance space. Additionally, foam bubbles generated at the surface of water can also flow through the clearance space. The collar 141 is surrounded by a tray 145 into which foam bubbles passing through the clearance space can spill and be contained as a bubble mass. The fine protein fraction of the biological waste in the water is removed on the surface of the air bubbles. Effluent from the bubble mass contained in the tray 145 can be disposed of in any appropriate way.

[0078] Each contact chamber 121, 122 contains biomedia 147, with the water contacting the biomedia as it passes through the respective contact chamber. The biomedia 147 may be of any appropriate type, such as that of a similar construction to the biomedia contained within the first biological filter 69.

[0079] Water to undergo biological filtration enters the biological filter 111 through inlet 115 and flows upwardly within the first contact chamber 121 towards the duct 120, with the water contacting the biomedia 147 during the upward flow. The water is aerated during the upward flow by way of the aeration means 131. The injected air strips the water of fine particles and the fine protein fraction of the biological waste in the water is removed on the surface of air bubbles which collect in the tray 145 of the protein skimmer 137. The water leaves the first contact chamber 121 through the flow path defined by the duct 120 and enters the second contact chamber 122 to flow downwardly towards the outlet 117, with the water contacting the biomedia 147 during the downward flow. The water undergoes a similar biological filtration process in the second contact chamber 122 as that to which it was subjected in the first contact chamber 121. There is, however, a difference in the filtration processes in the two contact chambers 121, 122. In the first contact chamber 121, the upwardly flowing water is moving in the same direction as the air bubbles rising therein. In contrast, the downwardly flowing water in the second contact chamber 122 is in counter-flow with the air bubbles rising therein. It is believed that the combination of uni-directional flow and counter-flow of the water and air bubbles enhances the filtration process.

[0080] The residence time of water within the each contact chamber 121, 122 is, of course, dependent on the size of the contact chamber and the water flow rate.

[0081] The biological filtration process conducted in the second biological filter 111 is almost exclusively of an aerobic nature. The process is performed by bacteria that converts the remaining ammonia (NH₃) to nitrite (NO₂) and all the nitrite (NO₂) is then converted into a non-toxic nitrate (NO₃). The bacteria performing the second process are from the Nitrobacter species.

[0082] The air supply means 27 include an air supply circuit 151 for supplying filtered air to various devices in the aquaculture system 10, including the air lock 18, the air injection UV steriliser 46, the air lift system 59, and the aeration means 131 of the second biological filter 111. The air supply circuit 151 includes an air pump 153 for delivering filtered air to the various devices. The air supply line 61 for the lift system 59 also forms part of the air supply circuit 151.

[0083] From the foregoing, it is evident that the present embodiment provides a aquaculture system for growing seafood in a closed environment in a sense that matter entering the environment is controlled to ensure that it is of an appropriate quality so as to avoid exposure of the seafood to disease and other harmful effects. Specifically, personnel and materials used in the aquaculture process enter and leave the sealed environment 13 via the airlock, with appropriate techniques being adopted to avoid contamination of the sealed environment 13 through the movement of personnel and materials. Air entering the sealed environment is filtered. The filtered air is used for various purposes including the airlock 18, the air lift system 59, the aeration processes involved in biological filtration, and for air injection in the UV water sterilisation process. The water employed within the culture tank 35 is initially sterilised, and subsequently filtered and sterilised on an ongoing basis during the aquaculture process. The filtration involves both mechanical and biological filtration, as previously described. The thermal conditions when the sealed environment 13 are maintained at appropriate levels by the temperature control means 31.

[0084] Various processes within the sealed environment 13 can be controlled automatically as appropriate, such as by way of a computer control system. Furthermore, the aquaculture system 10 can incorporate various monitoring and alarm devices so that remedial action can be taken promptly in the event of a malfunction or disturbance which might jeopardise the aquaculture process.

[0085] A particular feature of the aquaculture system according to the embodiment is that the plant used in establishing and maintaining the sealed environment 13 (including for example the culture tank 35, the water treatment system 41, and the air supply means 27) is contained almost entirely within the confines of the building 11. Furthermore, access to and from the sealed environment is managed by way of the access system 17 incorporating the air lock 18, for the purpose of avoiding contamination. Additionally, effluent arising from the aquaculture process is discharged from the sealed environment. Thus, the possibility of contamination of the sealed environment 13 is dramatically reduced.

[0086] It should be appreciated that the scope of the invention is not limited to the scope of the embodiment described. In particular, while the embodiment has been described in relation to cultivation of seafood such as fish, it should be appreciated that the invention may be equally applicable to cultivation of other appropriate aquatic species.

[0087] Furthermore, it should be appreciated that the mechanical filtration apparatus and the biological filtration apparatus employed in the aquaculture system according to the embodiment may each have applications in other aquaculture systems and also in fields outside of aquaculture.

[0088] Improvements and modifications may be incorporated without departing from the scope of the invention.

[0089] Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. 

1. An aquaculture system comprising a cell defining a closed environment, the cell having an access system incorporating an airlock for access to and from the closed environment, at least one culture tank accommodated in the closed environment, and water treatment means for treating water in the or each culture tank for removing contaminants therein to maintain the water of a quality appropriate to the species under culture.
 2. An aquaculture system according to claim 1 further comprising a water supply means for introducing water of a quality appropriate to the species under cultivation into the environment.
 3. An aquaculture system according to claim 1 or 2 further comprising air supply means for introducing air of a quality appropriate to the species under culture into the closed environment.
 4. An aquaculture system according to claim 1, 2 or 3 further comprising power supply means for introducing electrical power to the closed environment.
 5. An aquaculture system according to any one of claims 1 to 4 wherein the water treatment means comprises a plurality of filtration devices in a serial arrangement, water being extracted from the or each culture tank and contacted by the filtration devices for filtering thereof prior to being returned to the culture tank or tanks.
 6. An aquaculture system according to claim 5 wherein the filtration devices provide mechanical filtration and biological filtration.
 7. An aquaculture system according to claim 5 or 6 wherein the filtration devices include a mechanical filtration apparatus comprising a moveable structure incorporating a filter screen, means for contacting a portion of the filter screen with the water, and means for periodically moving the filter screen for varying the portion thereof being contacted by the water.
 8. An aquaculture system according to claim 7 wherein the filter screen comprises a plurality of screen sections disposed along a direction of movement of the moveable structure.
 9. An aquaculture system according to claim 7 or 8 wherein the moveable structure comprises an endless belt.
 10. An aquaculture system according to claim 7, 8 or 9 wherein the means for contacting a portion of the moveable structure with the water comprises a receiving chamber into which the water can be delivered and from which the water can flow under gravity through the filter screen.
 11. An aquaculture system according to any one of claims 7 to 10 wherein the mechanical filtration apparatus further comprises a cleaning means for cleaning a respective portion of the filter screen after that portion has moved out of contact with the liquid.
 12. An aquaculture system according to claim 11 wherein the cleaning means comprises a washing system for applying a washing liquid to the respective portion of the filter screen.
 13. An aquaculture system according to claim 12 wherein the cleaning liquid comprises water applied as a spray.
 14. An aquaculture system according to any one of claims 5 to 12 wherein the filtration devices include a biological filtration apparatus comprising a contact chamber, aeration means for introducing air into water contained in the chamber, and a protein skimmer for collecting foam bubbles generated by aeration of the water, the protein skimmer comprising a body defining a tray for receiving foam bubbles.
 15. An aquaculture system according to claim 14 wherein the tray surrounds a collar disposed about an opening extending through the body, the opening accommodating a delivery pipe of the aeration means with a clearance space therebetween.
 16. An aquaculture system according to claim 14 or 15 wherein the biological filtration apparatus further comprises a further contact chamber, a further aerating means for introducing air into a body of liquid contained in the further contact chamber, and a further protein skimmer wherein the two contact chambers are connected in series in a liquid flow circuit such that water undergoing biological filtration passes upwardly through one contact chamber and downwardly through the other contact chamber.
 17. An aquaculture system according to claim 16 wherein the liquid flow circuit has an inlet in one contact chamber and an outlet in the other contact chamber, the inlet and outlet each opening onto the respective contact chamber in a lower region thereof.
 18. An aquaculture system according to any one of claims 6 to 17, wherein the biological filtration is performed by a first biological filter and a second biological filter.
 19. An aquaculture system according to claim 18 wherein the first biological filter is disposed beneath the mechanical filtration apparatus to receive water directly therefrom.
 20. An aquaculture system according to claim 18 or 19 wherein the second biological filter comprises said biological filtration apparatus.
 21. An aquaculture system according to any one of the preceding claims wherein the water treatment means comprises a sterilisation system for sterilising the water as appropriate.
 22. A mechanical filtration apparatus comprising a moveable structure incorporating a filter screen, means for contacting a portion of the filter screen with a liquid requiring filtering, and means for periodically moving the filter screen for varying the portion thereof being contacted by the liquid.
 23. A mechanical filtration apparatus according to claim 22 wherein the filter screen comprises a plurality of screen sections disposed along a direction of movement of the moveable structure.
 24. A mechanical filtration apparatus according to claim 22 or 23 wherein the moveable structure comprises an endless belt.
 25. A mechanical filtration apparatus according to claim 22, 23 or 24 wherein the means for contacting a portion of the moveable structure with the liquid comprises a receiving chamber into which the liquid can be delivered and from which the liquid can flow under gravity through the filter screen.
 26. A mechanical filtration according to any one of claims 22 to 25 further comprising a cleaning means for cleaning a respective portion of the filter screen after that portion has moved out of contact with the liquid.
 27. A mechanical filtration according to claim 26 wherein the cleaning means comprises a washing system for applying a washing liquid such as water to the respective portion of the filter screen.
 28. A biological filtration apparatus incorporating a protein skimmer, characterised in that the protein skimmer is disposed above a body of liquid undergoing biological filtration.
 29. A biological filtration apparatus comprising a contact chamber, aeration means for introducing air into a body of liquid contained in the chamber, and a protein skimmer disposed above the body of liquid for collecting foam bubbles generated by aeration of the liquid.
 30. A biological filtration apparatus according to claim 29 wherein the protein skimmer comprises a body defining a tray for receiving foam bubbles.
 31. A biological filtration apparatus according to claim 30 wherein the tray surrounds a collar disposed about an opening extending through the body, the opening accommodating a delivery pipe of the aeration means with a clearance space therebetween.
 32. A biological filtration apparatus according to claim 29, 30 or 31 comprising a further contact chamber, a further aerating means for introducing air into a body of liquid contained in the further contact chamber, and a further protein skimmer disposed above the body of liquid, wherein the two contact chambers are connected in series in a liquid flow circuit such that liquid undergoing biological filtration passes upwardly through one contact chamber and downwardly through the other contact chamber.
 33. A biological filtration apparatus according to claim 32 wherein the liquid flow circuit has an inlet in one contact chamber and an outlet in the other contact chamber, the inlet and outlet each opening onto the respective contact chamber in a lower region thereof.
 34. A water treatment system comprising a water circuit including a biological filter, a pump disposed on the downstream side of the biological filter, and an air-injected UV steriliser disposed on the downstream side of the pump.
 35. An aquaculture system substantially as herein described with reference to the accompanying drawings.
 36. A mechanical filtration apparatus substantially as herein described with reference to the accompanying drawings.
 37. A biological filtration apparatus substantially as herein described with reference to the accompanying drawings. 